This invention relates to a process for separating dynamic and static components of a sequence of images.
In image processing techniques, when a series of images are obtained for a variable phenomenon (for example a meteorological or biological phenomenon) taken by an appropriate sensor (weather radar, radiographic instrument, etc.) at different times but under the same conditions (fixed coordinate system), these images may comprise dynamic components characteristic of the variation of the phenomenon concerned, and static components (which do not move in space, or which at least move in space at a negligible displacement speed compared with the speed of the dynamic components). For example in the case of meteorology, these static components may include ground relief elements (mountains, buildings, etc.), and in biological applications they may include elements of the skeleton or fixed objects located on the X-ray path, etc. In most cases, the dynamic component is the only part that is interesting in the images, consequently it should be possible to eliminate the static component not only because it is not useful, but also because it may make it significantly more difficult to observe the dynamic component with which it may become confused and can even hinder this observation. However in some applications, the static component may be useful, or both of these two components may be useful, in which case it may be necessary to observe them separately. Consequently in this type of case, it must be possible to separate these two components and if necessary to eliminate one of them. Processes based on different principles are known for eliminating radar image “clutter”, but they cannot make a sufficiently precise distinction between the dynamic and static components of the images obtained, particularly when these two components become coincident.
The purpose of this invention is a process for separating the static component from the dynamic component in a series of images of variable phenomena recorded under the same conditions at different times, so that one of them may be eliminated if necessary, in the most precise and reliable manner possible.
The process according to the invention consists of recording the intensity of each pixel in at least one area in the sequence of images taken at different times and under the same conditions, each time considering the same pixels in the different images, and then making use of pixel intensity sort criteria to discriminate between pixels for which the intensity varies much less than others during the sequence of images, the first pixels forming part of the static component of the images.
According to various aspects of the process according to the invention, one possible first criterion for classifying the intensity of pixels is their minimum. A second possible criterion for classifying them is the σ/m ratio of the variation in the pixel intensity, where σ is their standard deviation and m is their average. A third possible sort criterion is the spatial gradient of pixel intensities, that is included on the sequence of images with time, this criterion being used as a stop criterion for an active geodetic contour method, known in itself and described in research report No. 1075, 1989 by L. COHEN from the INRIA entitled “On active contour models”.